Attributes fix for percona-xtradb-cluster on RHEL

On RHEL distros it's Percona-XtraDB-Cluster-55 intead of percona-xtradb-cluster-55, so I've updated attributes/default.rb. Without that, on RHEL distros you will get:

ERROR: package[percona-xtradb-cluster-55] (percona::cluster line 31) had an error: Chef::Exceptions::Package: No version specified, and no candidate version available for percona-xtradb-cluster-55